The invention relates to a coupling comprising two parts supported one inside the other, which are rotatable relative to one another around a common longitudinal axis. Together, the parts form a chamber which is filled with a highly viscous fluid and which is sealed by an axially movable piston which is in a rest position when the two parts do not rotate relative to one another. In the chamber, between a pair of surfaces associated with the one and the other of the parts rotatable relative to one another, there is formed at least one circumferentially extending shear channel in which, in the case of a relative rotation between the parts rotatable relative to one another, pressure is built up in the fluid. The pressure loads the piston. Further, a friction coupling has friction elements which are connected to the one and the other two parts in a rotationally fast and displaceable way, and a stop part which is axially firmly associated with one of the rotatable parts, to achieve torque transmission between the rotatable parts as a function of a relative rotation.
Couplings of this type are offered by the Applicant under the name of Visco Lok couplings and have already been described in DE 43 27 519.2 and DE 43 43 307.3. On the one hand, they serve as differential locks in differential drives such as they are used in driving axles in the form of axle differentials or between two driving axles as distributor differentials. On the other hand, they can also be used on their own in the driveline of a motor vehicle wherein--only if there exists a speed differential between the axles of the vehicle--an additional axle is temporarily to be provided with torque (hang-on solution).
As the couplings do not increasingly build up a locking effect unless there exists a speed differential, their locking effect equals zero when starting the vehicle with all wheels in the stationary condition, so that it is quite possible for one wheel initially to spin if its friction coefficient relative to the road surface is different from the other ones (.mu. split). To avoid such wheel spin, it has already been proposed to provide a spring assembly which already loads the friction elements with a pretension when the piston is in a position of rest, so that there exists a locking effect even when the speed of all wheels is zero. However, such an assembly is disadvantageous in that it considerably aggravates slow vehicle maneuvering in the course of which it is typical for speed differentials to occur between all the steered wheels or between the steered wheels and the unsteered wheels.